The present invention relates to commercial and institutional kitchen exhaust systems, and more particularly, to an exhaust rate control method and apparatus for such exhaust systems.
Commercial and institutional kitchens are equipped to prepare food for large numbers of people and may form part of or adjoin larger facilities such as restaurants, hospitals and the like. Such kitchens are typically equipped with one or more commercial duty cooking units capable of cooking large amounts of food. On such a scale, the cooking process may generate substantial amounts of cooking heat and airborne cooking by-products such as water vapor, grease particulates, smoke and aerosols, all of which must be exhausted from the kitchen so as not to foul the environment of the facility. To this end, large exhaust hoods are usually provided over the cooking units, with duct work connecting the hood to a motor driven exhaust fan located outside the facility such as on the roof or on the outside of an external wall. As the fan is rotated by the motor, air within the kitchen environment is drawn into the hood and exhausted to the outside atmosphere. In this way, cooking heat and cooking by-products generated by the cooking units follow an air flow path defined between the cooking units and outside through the hood to be exhausted from the kitchen before they escape into the main kitchen environment and perhaps into the rest of the facility.
In many conventional installations, the motor driving the exhaust fan rotates at a fixed speed. The exhaust fan thus rotates at a fixed speed as well and, therefore, tends to draw air through the hood at a constant or fixed volume rate. However, the amount of cooking heat and/or cooking by-products generated by the cooking units will vary widely over the course of the day. It has been the practice in such instances to select a speed for the fan that will cause the system to exhaust a fixed volume rate of air based on the level of cooking heat and/or cooking by-products expected to be generated during anticipated peak usage of the cooking units. If the volume rate selected is too low, there will be times when the quantity of cooking by-products being generated exceeds the exhaust rate of the exhaust system. In such circumstances, the system will be in a relative underexhaust state such that cooking by-products will be released into the kitchen. The fixed volume rate is thus selected to be sufficiently large that under most normal operating situations, all of the cooking by products, for example, will be expelled out of the hood rather than released into the kitchen. As a consequence, during non-peak times, the exhaust fan is running faster than required so it tends to be in an overexhaust state wherein the volume rate of air being expelled is more than is necessary to clear the cooking by-products from the kitchen. In many exhaust conditions, as air is expelled through the hood, other air is drawn into the kitchen, such as from a make-up air system or the rest of the facility, which in turn draws in air from outside the facility. The heating, ventilating, and air conditioning ("HVAC") system of the facility must typically condition the drawn-in air. During overexhausting, the HVAC system may be heavily taxed to condition the drawn-in air. Thus, overexhausting has generally been recognized as uneconomical due to increased power usage by the exhaust system, reduced life of components such as the exhaust fan motor, and increased load on the HVAC system.
In order to prevent uneconomical overexhausting, I developed a system by which to vary the speed of the exhaust fan in accordance with the level of heat and/or by-products being generated by the cooking units. Such a system is described in my U.S. Pat. No. 4,903,685, the disclosure of which is hereby incorporated by reference in its entirety. In that system, when little or no cooking is occurring such that the level of heat, for example, being generated by the cooking units is extremely low, the speed of the fan is held low to expel air from the kitchen at a low volume rate. As cooking increases, the level of cooking heat also increases, and the speed of the fan is increased to increase the volume rate of air expelled from the hood to the outside. Consequently, the volume rate of air being expelled is generally proportional to the level of cooking heat being generated. The system may additionally, or alternatively, vary the volume rate in correlation to the level of cooking by-products being generated by the cooking units. In some situations, when any cooking by-product is detected, the exhaust volume rate may be forced to a high level, such as maximum, irrespective of the cooking heat level or variations in the level of cooking by-product. Varying the volume rate of air exhausted is expected to generally improve the energy efficiency of the facility. The foregoing notwithstanding, varying the volume rate solely based on the activity of the cooking units fails to account for opportunities to improve the comfort or enhance safety in the kitchen or other parts of the facility.
By way of example, there are typically substantial periods of time during which little or no cooking is being undertaken. During these idle times, the volume rate of air being exhausted will typically be quite low or even zero. Nonetheless, an ambient air environment away from the hood and air flow path but within the main area of the kitchen can still become quite hot. A typical HVAC system may require significant amounts of energy to cool the kitchen down to a more comfortable level and could also cause the rest of the facility to become uncomfortably cold. Conversely, as the HVAC system heats the facility, the kitchen may be caused to become uncomfortably hot. Similarly, the ambient air environment may become uncomfortable and/or unsafe due to build up of noxious gases or other harmful agents. For example, carbon dioxide may increase in the ambient air environment, particularly in the dining room, for example, due to the number of occupants of the facility. The above problems can also be encountered during non-idle times such that exhausting at a volume rate sufficient to exhaust cooking heat, for example, will not be sufficient to cool the kitchen or clear noxious gases.